Design high-precision industrial sensing system front end

06/13/2019

Industrial and process control applications collect large amounts of accurate temperature, pressure and strain data for upstream decision making. The challenge for designers is that these applications require multiple high-precision channels to maintain high accuracy in the frequency domain.


This article discusses the key components and parameter requirements for accurate, high-performance industrial sensing and signal conversion front ends. Since noise is the determinant of accuracy, the ultimate solution is to solve the noise problem.


System overview

High-precision 18-bit industrial sensing front-end systems should include a cost-effective, isolated multi-channel data acquisition (DAQ) architecture that manages industrial signal levels. From input to output, the multichannel precision circuit to be described starts with an eight-input multiplexer and can be configured as a single-ended or differential input channel (Figure 1). These multiplexer inputs receive various sensor inputs for process control, such as inputs from temperature, pressure, and optical sensors.


Eight input, multi-channel, high precision circuit diagram

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Figure 1: Eight-input, multi-channel precision circuit for multiple sensor inputs. The input multiplexer can be configured as a single-ended or differential input channel. (Source: Bonnie Baker)


In Figure 1, the Programmable Gain Instrumentation Amplifier (PGIA), denoted "PGA", follows the input multiplexer and has similar input and output swing voltage capabilities. Both the multiplexer and the PGIA stage are capable of managing high voltage inputs up to ±10 volts.


The PGIA's common-mode voltage and wide-voltage output swing are inconsistent with the single-supply input range of an 18-bit analog-to-digital converter (ADC). In order to prepare the signal voltage range of the ADC, the system requires a funnel amplifier. The funnel amplifier performs three functions: signal level shifting, single-ended to differential conversion, and attenuation to meet the input requirements of a single-supply 18-bit ADC.


Digital isolators provide galvanic isolation after an 18-bit ADC. This isolation allows for different common-mode voltages between each side without interfering with signal fidelity.


Circuit details

As mentioned above, the isolated multi-channel DAQ system has a multiplexer, PGIA stage, ADC amplifier driver and precision fully differential successive approximation register (SAR) ADC. The system uses a single ADC to monitor 8 channels. However, the ADC driver and ADC are the main noise contributors (Figure 2).


Schematic of isolated multi-channel DAQ system with 18-bit ADC (click to enlarge)

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Figure 2: shows a schematic of an isolated multi-channel DAQ system with an 18-bit ADC. The ADC and ADC drivers are the main noise contributors. (Source: Analog Devices)


The noise level is a specification that specifies the type of component that is appropriate for the application circuit.


Choose the right component

In Figure 2, the input multiplexer is Analog Devices' ADG5207BCPZ-RL7, a high voltage, latch-proof, 8-channel differential multiplexer with 3.5 picofarads (pF) of ultra-low capacitance and Charge injection of 0.35 pico library (pC). This low charge injection makes these switches ideal for sample-and-hold DAQ circuits that require low glitch and fast settling times. The ADG5207 can be configured to accept both single-ended and differential input signals. The Complex Programmable Logic Device (CPLD) shown in the circuit selects the active channel of the ADG5207 by using its address pins.


PGIA is the AD8251ARMZ-R7 from Analog Devices. The device offers selectable gains of 1, 2, 4 and 8. Then, Analog Devices' AD8475ACPZ-R7 selectable gain fully differential funnel amplifier provides a 2.048 volt level shift for the ground common-mode voltage with gain settings of 0.4 and 0.8. The AD8475 has a low output noise spectral density of 10 nanovolts per square Hertz (nV / √ Hz). The combination of the gain of the PGIA and the funnel amplifier provides the appropriate full-scale input signal for the Analog Devices AD4003BCPZ-RL7 18-bit SAR ADC (Table 1).

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Table 1: Input and output voltage ranges correspond to the four gain configurations of the AD8251 PGIA. The combination of the gain of the PGIA and the AD8475 funnel amplifier provides the appropriate full-scale input signal for the AD4003BCPZ-RL7 18-bit SAR ADC. (Source: Bonnie Baker)


The AD4003BCPZ-RL7 is a fully differential, 2 megasamples per second (MSPS), 18-bit precision SAR ADC with a typical signal-to-noise ratio (SNR) of 98 decibels (dB) for a 4.096 V reference.


System noise analysis

Due to its impact on accuracy, noise must be carefully considered when designing higher speed precision DAQs. Noise is a phenomenon in the frequency domain that affects the AC and DC accuracy of the ADC's digital output. Noise is a random event: a noise circuit can provide absolutely correct results for a single conversion, and the next conversion can produce very inaccurate results. The challenge for designers is to determine the acceptable noise contribution of all devices in the circuit.


The total system root mean square (rms) noise is equal to the root and square of all devices in the circuit referenced to the AD4003 ADC input and is calculated using Equation 1:


Formula 1 formula 1


where:


V n ADG5207 = ADG5207 multiplexer rms noise contribution


V n AD8251 = AD8251 PGIA rms noise contribution


V n AD8475 = rms noise contribution of the AD8475 funnel amplifier


V n AD4003 = AD4003 18-bit ADC rms noise contribution


The calculated system rms SNR uses the full-scale input range of the AD4003 or V REF and is calculated using Equation 2:


Formula 2 formula 2


AD4003 ADC Noise: The AD4003 ADC noise is a function of converter quantization error and internal thermal noise. According to Equation 3, the AD4003 rms input voltage noise is calculated using the full-scale input voltage (V REF) and the operating SNR:


Formula 3 formula 3


The data sheet specification REF for the AD4003 is equal to 4.096 volts and is approximately 98 dB.


AD8475 Funnel Amplifier Noise: The AD8475 rms output noise is a combination of the spectral noise density of the amplifier at 1 kilohertz (kHz) (εAD8475) and the bandwidth limitations of the amplifier circuit. The AD8475 with a gain of 0.4 V / V has a bandwidth equal to 150 MHz (MHz). The following resistor-capacitor (RC) filter has a 3 dB corner frequency of 6.63 MHz. According to Equation 4, the combination of the AD8475 and the output RC filter produces a bandwidth limit of 6.63 MHz:


Formula 4 formula 4


where:


ε AD8475 = 10 nanovolts / √ Hz.


R = 200 ohms (Ω)


C = 120pF


BW RC = 1 / (2xp × R × C) ~ 6.63MHz


AD8251 PGIA noise: The rms noise contribution of the AD8251 is a function of its reference input AD8251, 1 kHz point noise (εAD8251) in nV /√Hz, its gain setting (G AD8251), gain of AD8475 (G AD8475) and Noise filter bandwidth at the input of the AD4003 (BW RC). It is calculated using Equation 5:


Formula 5 Equation 5


For a gain of 1 V / V, the value of εAD8251 is equal to 40 nV /√Hz, and for a gain of 8 V / V, the value of εAD8251 is equal to 18 nV /√Hz.


ADG5207 Multiplexer Noise: The Johnson-Nyquist noise equation provides the noise spectral density of the multiplexer and the resulting rms noise, Equation 6:


Formula 6 Equation 6


where:


k B = Boltzmann constant = 1.38 x 10 -23


T = temperature in Kelvin


R ON = multiplexer "on" resistor (according to the ADG5207 data sheet)


Using this formula (Equation 6) is suitable because the multiplexer acts like a series resistor.


The multiplexer's spectral density value (εnADG5207) uses Equation 7 to derive the ADG5207 rms noise contribution:


Formula 7 Equation 7


Noise analysis summary

The resulting calculated noise contribution for each component in Figure 2 and the resulting SNR of the cumulative gain of 3.2 are 84.7 dB. The most important factors for total noise are the AD8251 PGIA and AD4003 ADC (Table 2)

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Table 2: The calculated SNR performance of a multi-channel DAQ system with a cumulative gain of 3.2 is 84.7 dB. (Source: Analog Devices)


Circuit evaluation and testing

To evaluate and test the circuit, designers can use the EVAL-CN0385-FMCZ circuit evaluation kit, which includes the circuit in Figure 2 (Figure 3).


Picture of Analog Devices EVAL-CN0385-FMCZ Evaluation Board

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Figure 3: The EVAL-CN0385-FMCZ evaluation board can be used to test the DAQ front-end design described in this article. (Source: Analog Devices)


The CN-0385 Design Support Package contains complete schematics and layout support materials. The evaluation kit also includes the EVAL-SDP-CH1Z controller board for easy data acquisition (Figure 4).


Test setup function layout for evaluating the DAQ front end

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Figure 4: Test setup feature layout to evaluate the DAQ front end. (Source: Analog Devices)


The performance results for the EVAL-CN0385-FMCZ board show values that closely match the noise calculation (Table 3).


Cumulative gain Signal-to-noise ratio (dB) Noise (μV RMS) THD (dB)

0.4 93.9 55.2 -99.2

0.8 92.8 62.6 -98.5

1.6 90.6 80.7 -97.0

3.2 88.0 108.9 -94.6

Table 3: SNR, Noise, and Total Harmonic Distortion (THD) performance of the EVAL-CN0385-FMCZ board with cumulative gains of 0.4, 0.8, 1.6, and 3.2 for a 10 kHz full-scale sine wave input. (Source: Analog Devices)


The audio precision SYS-2700 generates the signal as a differential input mode. A 10 kHz input signal Fast Fourier Transform (FFT) plot is shown (Figures 5, 6, 7 and 8).


FFT plot for 10 kHz, 20 volt pp input

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Figure 5: FFT at 10 kHz, 20 V pp input, gain on a single static channel = 0.4. (Source: Analog Devices)


FFT plot for 10 kHz, 10 volt pp input

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Figure 6: FFT at 10 kHz, 10 V pp input, gain on a single static channel = 0.8. (Source: Analog Devices)


FFT plot for 10 kHz, 5 volt pp input

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Figure 7: FFT at 10 kHz, 5 V pp input, gain on a single static channel = 1.6. (Source: Analog Devices)


FFT plot for 10 kHz, 2.5 volt pp input

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Figure 8: FFT of a 10 kHz, 2.5 volt pp input on a single quiescent channel with gain = 3.2. (Source: Analog Devices)


As shown, the performance of the ADG5207, AD8251, AD8475, and AD4003 signal chains in the EVAL-CN0385-FMCZ evaluation board is very close to the early calculations.


in conclusion

In industrial and process control environments, there is a large amount of data collection activity, including the collection of accurate temperature, pressure and strain data. These applications require multiplexed, high-precision channels while maintaining high accuracy and low noise in the frequency domain. The ideal analog measurement front end features a multiplexer PGIA and an 18-bit, 2.0 MSPS precision ADC. The ADC samples the signal from the active multiplexer channel. This article provides accurate calculations and additional test data for the appropriate circuit. The test results show that the actual performance of the ADG5207, AD8251, AD8475 and AD4003 signal chains in the EVAL-CN0385-FMCZ evaluation board is very close to the calculated value.